During the process of drawing an optical fiber from a glass preform, there is an ongoing need to accurately characterize the dimensions of the fiber in order to understand, predict and control its properties and their effects on signal propagation. Additionally, during the fabrication of the preform itself, it is desirable to characterize and control the dimensions of various layers being deposited on or in a preform tube. For example, when using an MCVD technique, there is a need to continuously monitor the inner diameter of the glass tube so that the amount of deposited soot can be accurately determined.
The conventional techniques used today to provide dimension characterization during optical fiber fabrication either exhibit a relatively low degree of accuracy or cannot be easily implemented in a manufacturing environment for continuous, on-line monitoring and control.
Most draw towers use commercial devices based on a technique referred to as “shadowing” for measuring fiber diameter during the draw process (or preform inner diameter during soot deposition). In this technique, a laser light is scanned across the target object (fiber or preform), where the change in transmission is then analyzed to retrieve the dimension of the target object. In most cases, the accuracy of this technique is on the order of about 0.1 μm—and requires averaging, where the averaging makes this technique relatively slow. Moreover, this technique requires calibration and cannot detect the presence of air pockets that may arise during the fiber draw process.
An alternative interferometric technique, referred to in the art as “FOCSL”, which stands for “Fiber Optic Characterization by Scattering Light”, counts the number of fringes that are produced due to interference between the light that is transmitted through an object and the light that is reflected off of the object. In comparison to the above-mentioned technique, FOCSL exhibits an accuracy on the order of about 0.05 μm. Other interferometric techniques exist in which a single frequency is scanned across the object and the phase difference with and without the object is measured to determine the object's diameter.
Most interferometric techniques are capable of measuring only the optical thickness, which is a product of the refractive index and the physical thickness. The physical thickness is determined by assuming the value of the refractive index and dividing this value by the measured optical thickness. However, there are cases when the refractive index is not known very accurately, such as in the case during thin film growth. It is therefore desirable to have a technique available that is able to measure both refractive index and physical thickness independently.
In most cases, optical fibers are coated with a dual (primary and secondary) polymer coating. The bend loss of the fiber depends on the primary coating thickness. Currently, there is no way to monitor the primary coating diameter while the fiber is still in place on the draw apparatus.
The available prior art measurement techniques all rely on analyzing light that is forward scattered through the fiber/preform, and therefore requires that detectors and light collection optics be placed on the drawing apparatus. This requirement is often cumbersome due to space limitations associated with most draw towers.
Thus, a need remains in the art for an arrangement capable of characterizing an optically transparent object, such as a fiber or preform, which is continuous in nature, accurate, and can be located remotely from the drawing apparatus.